The problem is a general one and it can be reproduced with standard asp
controls and without any RadControls as well. What actually happens is
that when the RadWindow is added to the RadWindowManager it is actually
moved in its Controls collection and this happens in the
CreateChildControls event of the manager while the update
panel registers itself in the Init event which is earlier in the
lifecycle. More information about this is available here
Since the resolution of the issue is to add the RadWindow in the PreInit
event this cannot be fixed in the source code because it is to early to
add the RadWindow there and will affect the current functionality. In such setup we can suggest the following two simple solutions which fix the issue and will lead to the desired result:
1) You can declare a separate RadWindow directly on the page instead of
in the RadWindowManager - it will function as expected and there is no
limitation or additional load to use the control separately.
2) You can use RadAjaxManager for ajaxifying.